The present invention refers to a drive unit for a centrifuge rotor of a centrifugal separator, wherein the drive unit includes a drive shaft, which is arranged to carry said centrifuge rotor and is rotatably journalled in at least one bearing member, and a casing, which delimits an upper space, in which the drive shaft and said first bearing member are located, and a lower space for storing liquid oil, and wherein the drive unit includes first means, which are arranged to create a flow of air and said oil for forming an oil mist and to bring this oil mist to flow through said bearing member for cooling and lubricating thereof.
The cooling of bearings of known drive units for centrifugal separators frequently rely on an internal circulating air stream, to which are supplied droplets of oil from an oil pan in a lower space in the drive unit in such a way that an oil mist is formed. The air stream with the oil mist is circulated through the bearings by means of a fan or similar member, and absorbs the heat formed in the bearing while at the same time lubricating the bearing. The air stream with the oil mist is further circulated after passing through the bearing in the inner space defined by the casing of the drive unit, wherein the heat absorbed by the oil droplets and the air is conducted away to other parts of the drive unit. A part of the oil droplets are thereby deposited onto the surfaces of the parts along which the oil flows down to the oil pan where it is collected. In the centrifugal separators now available, the heat release from the air stream to the separator walls is however not particularly efficient. U.S. Pat. No. 4,541,736 discloses a drive unit for a centrifuge rotor of a centrifugal separator. The known device is arranged to feed oil droplets in a stagnant air atmosphere through the bearing members carrying the rotor spindle.
The object of the present invention is to overcome the problems mentioned above and to improve the efficiency of the bearing cooling in a centrifugal separator.
This object is obtained by the drive unit initially defined, which is characterised by means, arranged to cool said oil mist by permitting transportation of a substantial part of said oil mist from an outlet of the bearing member through at least a passage extending in heat transferring contact with a surrounding space outside the casing to the lower space. By such an arrangement, an efficient heat release is obtained from the oil mist, which has flown through the bearing, i.e. the cooling of the oil is improved in comparison with the corresponding cooling of previously known centrifugal separators. Since the oil mist, upon presentations to the bearing has a relatively low temperature, an efficient bearing cooling may be obtained by a relatively small flow of the oil mist through the bearing.
According to an embodiment of the invention, said passage extends from an upper part of the upper space defined by the casing to a lower part of the upper space. Since the passage extends along a large part of the upper, free space, i.e. the space above the liquid oil, it is possible to ensure an efficient cooling of the oil mist. Thereby, said passage may extend along a substantial part of the length of the casing.
According to a further embodiment of the invention, said passage is formed at least partly by a wall member, which is in heat transferring contact with a surrounding space outside the casing. Such a wall member may be formed by the wall of the casing, i.e. said passage extends directly inside the casing or through channels in the wall of the casing. However, the passage may be formed in many different ways according to the present invention. For instance, the wall member may be formed conduits, which extend substantially outside the casing, wherein the oil is guided into the conduit through a passage in the casing at an upper part of the upper space and out from the conduit through a passage through the casing at a lower part of the upper space, i.e. immediately above the oil in the lower space.
According to a further embodiment of the invention, said second means include a first shielding member which extends outwardly from the bearing member in a direction towards an inner wall of the casing. In such a way the oil is forced away from the bearing and outwardly towards the relatively cold casing. Furthermore, said second means may advantageously include a second shielding member, which extends between the drive shaft and an inner wall of the casing for forming said passage. By such a shielding member a passage between the relatively cold casing and the shielding member is obtained. The second shielding member may extend substantially axially downwardly from a radially outer edge of the first shielding member. Furthermore, the second shielding member may extend so far downwardly that a gap is formed between a lower edge of the second shielding member and the liquid oil in the lower space. In such a way an efficient cooling of the oil is ensured and, moreover, a not insignificant part of the oil will be collected in the liquid oil in the lower space. The second shielding member may also extend at least partway around the drive shaft. In such a way, substantially the whole wall of the casing towards the surrounding space may be utilised as a cooling surface for the oil.
According to a further embodiment of the invention, the drive unit includes surface increasing members, for instance in the form of projecting flanges or the like, which are provided in said passage in order to increase the heat transfer from the oil, and/or surface increasing members, for instance in the form of projecting flanges or the like which are provided on the outer side of the casing up towards said surrounding space in order to increase the heat release to said surrounding space.
According to a further embodiment of the invention, said first means include a fan member which is arranged to drive one of said flow of air and oil through the bearing member and said passage. Thereby, heat from the bearing member will be transferred to the surrounding space via the air and the oil. Advantageously, the fan member is fixedly provided on the drive shaft.